In recent years, with advances in miniaturization of semiconductor integrated circuits, projection-exposure systems have been developed that use, instead of the ultraviolet (UV) light utilized by older projection-exposure systems, extreme ultraviolet (“EUV”) light. EUV light has substantially shorter wavelengths (for example, approximately 11 to 14 nm) than the UV light used previously. The shorter wavelength of EUV light improves the resolving power of optical systems that had reached their diffraction limits with respect to UV light. See Japan Laid-open Patent Document No. 2003-14893.
In configuring a projection-exposure system using EUV light (such a system is called an “EUV lithography” system, abbreviated “EUVL” system), there are no known materials that both transmit EUV light and exhibit sufficient refraction to such light to be useful as EUV lenses. Consequently, in an EUVL system, the constituent optical systems must be configured using EUV-reflective mirrors. But, in this wavelength range, the mirrors must be either oblique-incidence mirrors that utilize total reflection due to their refractive index being slightly smaller than 1, or multilayer-film mirrors in which the respective phases of multiple fronts of weakly reflected light at layer interfaces are superposed constructively in the reflected light to obtain high overall reflectance.
A EUV-reflective mirror employed in an EUVL system must be formed with a highly accurate and precise reflective-surface shape (surface “figure”) having extremely small figure errors with respect to wavefront aberration of reflected light. However, machining such a mirror is very difficult. Hence, techniques have been developed that are applied after the multilayer film has been formed on the reflective surface and that involve “shaving” away one layer at a time in selected regions of the multilayer-film reflective surface. This layer-shaving effectively corrects aberrations arising even from sub-nanometer figure errors. See International Patent Publication No. 01/41155.
In the case of a multilayer film comprising alternating layers of molybdenum (Mo) and silicon (Si), shaving the multilayer-film surface can result in an easily oxidized Mo layer being exposed to the atmosphere. Consequently, a single layer of Si or other oxidation-preventing substance, a ruthenium (Ru) layer, or other “capping” layer (to prevent oxidation of the exposed Mo layer) is normally applied at least in the shaved regions. The ruthenium (Ru) layer or other “capping” layer may also prevent carbon contamination in the multilayer film.
Because optical Ru layers are similar in many ways to Mo layers, depositing a Ru layer as a capping layer on the surface of a shaved multilayer film causes a considerable change in the phase of the reflected wavefront. The magnitude of the change depends upon the amount of film shaved away. The effect arises due to the fact that the Ru capping layer is conventionally formed over the entire surface, including in locations other than regions in which Mo-layer shaving has occurred. The capping layer also changes the reflectance of the shaved regions, causing irregularities in light propagation in the optical system.